The present invention relates to semiconductor devices and, more particularly, to semiconductor devices useful in the fabrication of light emitting diodes (LEDs).
Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. LEDs generally include an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted omnidirectionally from the active layer and from all surfaces of the LED. Recent advances in LEDs (such as nitride based LEDs) have resulted in highly efficient light sources that surpass the efficiency of filament-based light sources, providing light with equal or greater brightness in relation to input power.
Successful materials for producing LEDs (including light emitting diodes, laser diodes, photodetectors and the like) capable of operation in the UV, blue and green portions of the electromagnetic spectrum include the Group III nitride compound semiconductor materials, and in particular gallium nitride-based compound semiconductors. However, gallium nitride (GaN) presents a particular set of technical problems in manufacturing working devices. The primary problem is the lack of bulk single crystals of gallium nitride which in turn means that gallium nitride or other Group III nitride devices must be formed as epitaxial layers on other materials.
Sapphire (i.e., aluminum oxide or Al2O3) has been commonly used as a substrate for Group III nitride devices. Sapphire offers a reasonable crystal lattice match to Group III nitrides, thermal stability, and transparency, all of which are generally useful in producing a light emitting diode. Sapphire offers the disadvantage, however, of being an electrical insulator. This means that the electric current that is passed through an LED to generate the emission cannot be directed through the sapphire substrate. Thus, other types of connections to the LED must be made, such as placing both the cathode and anode of the device on the same side of the LED chip in a so-called “horizontal” configuration.
In contrast to sapphire, silicon carbide (SiC) can be conductively doped, and therefore can be effectively used to manufacture a “vertical” Group III nitride LED, in which ohmic contacts can be placed at opposite ends of the device. In addition, silicon carbide has a relatively small lattice mismatch with GaN, which means that high-quality Group III nitride material can be grown on it. Silicon carbide also has a high coefficient of thermal conductivity, which can be important for heat dissipation in high-current devices such as laser diodes.
Despite the advantages of these and other LED devices, the fabrication of LEDs using heteroepitaxial growth on substrates such as silicon carbide or sapphire can be problematic. Some crystal mismatch can occur when a GaN epitaxial layer is grown on a different substrate, such as a SiC substrate, and the resulting epitaxial layer can be strained by this mismatch. Such mismatches, and the strain they produce, can carry with them the potential for crystal defects, which in turn can affect the electronic characteristics of the crystals and the junctions and thus correspondingly can degrade or even prevent the performance of the device.
For example, threading dislocations, which are linearly extending defects that penetrate the crystal layer along the growth direction, can be introduced into the epitaxial layer during the process of lattice relaxation. If the deposited layer has many penetrating defects, the light emitting performance of the device can deteriorate substantially. Threading dislocations can act as a non-radiative recombination center for carriers, and accordingly the presence of such non-radiative centers can reduce device brightness and efficiency.
In addition, conventional silicon carbide substrates can absorb some light in portions of the visible spectrum. For silicon carbide devices that are vertical devices that are mounted with the substrates facing down, some light entering the substrate can be reflected back through the substrate before it is extracted from the device, thereby increasing absorption losses in the substrate. Reflection losses also may reduce the overall efficiency of the device.